Project Summary/Abstract Following an initial seizure, only some individuals develop temporal lobe epilepsy (TLE) but few methods exist to predict individual risk 1,2. Thus there is a pressing need to identify biomarkers and predisposing conditions for epileptogenesis. This pilot study aims to test our central hypothesis that vulnerability of the dentate gyrus (DG) predisposes individuals for epileptogenesis. TLE shares symptoms with other disorders (e.g.Alzheimer's disease (AD), autism spectrum disorder (ASD), schizophrenia and tuberous sclerosis), not only including seizures with temporal lobe foci but also problems with memory and navigation, confusion and social impairments 3-15. All of these diseases share pathology in the DG including cell loss, aberrant axonal sprouting, neurogenesis and inhibitory circuitry. The DG is part of the hippocampal formation in the temporal lobe, and is critical for navigation, episodic memory and acts as a gate or filter that limits propagation of excessive excitation passing from cortex to downstream hippocampal areas 16-18. The DG is hypothesized to be the main locus of the computational function of pattern separation that re-encodes similar memories into less similar output in order to a) avoid confusion between overlapping patterns and b) avoid hyperexcitation 19-22. Malfunction of pattern separation could therefore simultaneously result in epilepsy and associated cognitive problems. However, until recently, pattern separation has been studied exclusively through theoretical models or by behavioral assays, with no experimental demonstration that DG itself is the pattern separator. To remedy this knowledge gap, we developed a novel brain slice model in which patterns are fed into the DG via perforant path stimulation and their separation is quantified by measuring DG output 24. Thus, pattern separation can be observed in isolated hippocampal tissue, providing an accessible platform for dissecting its underlying mechanisms and possible failure during TLE. Here, we will use two well-studied mouse strains that have low (C57Bl/6) and high (DBA/2J) susceptibility to kainate-induced status epilepticus (SE) 27,28, in combination with behavioral testing, video-EEG and patch clamp electrophysiology in slices, to address the following Specific Aims: (1) - Determine the relationship between pattern separation memory, electrographic activity and circuit- level pattern separation. (2) - Determine key mechanisms contributing to circuit-level pattern separation and how they are altered in epilepsy. Comparison of DG-mediated pattern separation memory and DG circuit physiology between low- and high-susceptibility strains (with and without an insult) will reveal whether baseline differences in DG function are associated with a predisposition to seizures and whether changes in DG function correlates with epileptogenesis. 1